Carpal tunnel syndrome is at present the most widespread occupational health hazard in the industrial world, and it is receiving increasing attention from corporate as well as government agencies responsible for occupational health and safety. Many billions of dollars are consumed by this syndrome each year in lost working time and medical treatment. Carpal tunnel surgery is currently one of the most frequently performed surgeries in the United States. Although some of the physiological factors associated with idiopathic carpal tunnel syndrome are well studied and documented, including the increase in carpal tunnel pressure and the non-inflammatory changes in the synovium, the etiology of idiopathic carpal tunnel syndrome remains obscure, and is the subject of continued speculation and scientific study.
Carpal tunnel syndrome is associated with swelling of the carpal tunnel synovium. When synovium harvested at the time of carpal tunnel surgery is examined by conventional light microscopy techniques, it is remarkably normal in appearance and notable for the absence of inflammatory cells. This and other information suggests that the primary pathology is not intrinsic to the synovium itself, but that the synovium becomes swollen and thickened in response to mechanical stresses.
Normal synovial membrane is thin, elastic and compliant in nature. It is mechanically incapable of maintaining the anatomic relationships between the various digital flexor tendons in the carpal tunnel.
In anatomic areas other than the carpal tunnel, the biomechanical effect of each tendon is defined by a discrete fiberosseous tunnel that dictates the moment arm that the tendon utilizes to effect a rotational force or moment on the joint which the tendon is crossing. In the carpal tunnel, a similar stable relationship between the various tendons during wrist extension flexion can only be maintained if there is a synergistic tensioning of all the tendons to form a "dynamic pulley" capable of stabilizing each and every carpal tunnel tendon.
The profundus tendons embrace the dorsal wall of the carpal tunnel. The profundus tendons of the long, ring, and little fingers are cross-linked and thereby present a common muscle tendon unit that trifurcates at the distal end of the carpal tunnel sending separate tendons to the distal joints of the ulnar three fingers. In concert with the index profundus, these tendons mechanically fill the dorsal wall of the carpal tunnel, and as a functional unit, they present a smooth palmar surface to the overlying sublimus tendons and their associated synovial membranes. This surface is further enhanced as a continuous bearing surface by the third and fourth lumbrical muscles that arise from the adjacent sides of the long and ring, and the ring and little finger profundi. Additional support to the dorsal wall of the carpal tunnel is provided by the first and second lumbrical muscles that arise from the palmar radial sides of the index and long finger profundus tendons. The origin of these unique muscles migrate from a mid-palmar position, with the fingers extended, to a position just proximal to the level of the distal radius, with full finger flexion.
When a singular finger momentarily acts to press a key on a computer keyboard, if only its associated extrinsic flexor muscles contract, while the remaining carpal tunnel flexor-tendon units remain relaxed, and if the wrist position allows some degree of angulation, those tendons under tension will translate across the carpal tunnel. For example, with the wrist extended, the tension for index finger flexion creates a force that attempts to displace the sublimus tendon dorsally. If the dorsal extent of the carpal tunnel is stabilized by active tension in the profundus tendons with or without active contraction of their lumbrical muscles, then the sublimis tendon will be prevented from translating dorsally. Because each carpal tunnel tendon angulates as it extends to its respective digit from the distal end of the tunnel, there is no "safe" position of the wrist that will eliminate these tendon translating forces or even minimize the translation potential of each of them with a defined wrist position at any instant in time. Without the dynamic and synergistic action of all the carpal tunnel muscle tendon units and the lumbricals, the compliant synovium, being incapable of restraining the movement, will be subjected to an undesirable cyclic load including shear stresses and secondary swelling from the same.
These same principles would be operative with the wrist in flexion, and the more palmar sublimi, adjacent to the transverse carpal ligament, would need to actively contract to block palmar translation of the remaining sublimus and profundus muscle tendon units. This concept of synergistic muscle contraction to achieve carpal tunnel tendon stability applies equally to wrist deviations in the radial-ulnar plane or any combination of such deviations with flexion or extension.
Rapid alpha-numeric data entry via currently-available computer keyboards allows the use of relatively small actuating forces Minimal active stability of the distal joints is required, and can be accomplished with contraction of a single extrinsic muscle tendon unit, such as that of the sublimus. This action is further encouraged with longer fingernails, where the nail keeps the person from flexing the distal interphalangeal joint as they strike the keys with the palmar pulp rather than the distal pulp of the tip of the finger or its protruding nail.
The incidence of carpal tunnel syndrome has continued to increase dramatically since the current style electronic keyboard has come into wide usage. The keyboard of the old manual typewriter did not have a significant incidence of carpal tunnel syndrome associated with its usage. There are three significant differences between the old manual (i.e., mechanical) keyboard and the current electronic keyboards, namely, keystroke length, actuation force, (and its variable degree through the stroke length), and key height differential (front to back row key heights). Table 1 is a sample of these differences.
TABLE 1 ______________________________________ Median Median Stroke Height Diff. Force Manufacturer Type mm mm Kg ______________________________________ Sharp Electronic 1.8 14.2 0.044 Apple Electronic 1.9 9.7 0.070 IBM Electric 3.7 12.7 near zero Smith-Corona Electric 2.7 12.7 0.078 Olivetti Manual 14.7 26.9 0.910 ______________________________________
There has not heretofore been provided a keyboard structure which will teach, encourage or even require the keyboard operator to actively recruit the contraction of all (or most of) the muscle tendon units passing through the carpal tunnel in order to stabilize those tendons whose digits are actuating keys.